Hydration resistance for dolomite grain

ABSTRACT

The hydration resistance of a low-flux CaO . MgO refractory is increased by incorporating at least 0.01 percent by weight of a stabilizing agent selected from the group consisting of Al2O3, V2O5, aluminum fluoride, MgF2, and CaF2, and mixtures thereof to increase the total content of the stabilizing agent in the refractory grain to from 0.01 percent to about 1.0 percent by weight.

United States Patent 1191 Staut 1111 3,753,747 1451 Aug. 21, 1973HYDRATION RESISTANCE FOR DOLOMITE GRAIN [75] Inventor: Ronald Staut,Cherry Hill, NJ.

[73] Assignee: General Refractories Company,

Philadelphia, Pa.

[22] Filed: Oct. 1, 1971 [211 App]. No.: 185,869

[52] US. Cl 106/58, 106/61, 106/62,

- 106/63 [51] Int. Cl. C04b 35/04, C04b 35/06 [58] Field of Search106/61, 58, 62, 63

[56] References Cited UNITED STATES PATENTS 3,304,]88 2/[967 Hughey106/61 6/1941 Pitt et a1. l06/6l 8/1950 Hughey ..106/61 PrimaryExaminer-lames E. Po er Attorney-Ford F. Farabow, Brian G. Brunsvoldetal.

[57] ABSTRACT 10 Claims, No Drawings HYDRATION RESISTANCE FOR DOLOMITEGRAIN This invention relates to improving the hydration resistance of ahigh purity CaO MgO refractory grain. Most particularly, the inventionrelates to a process for producing hydration resistant and nearlytheoretically dense dolomite grain from a mixture of oxides containingat least 98 percent CaO MgO.

Burned dolomite is an attractive steel making refractory materialbecause of the high melting point of the CaO MgO mixture. The eutectictemperature for this system is 2,400 C. Accordingly, dead-burned orsintered dolomitic refractories prepared by high temperature calcinationand sintering of dolomitic ores are used extensively in the steelindustry for the repair of the hearth linings of basic electric andopen-hearth furnaces.

In addition, such refractories often comprise the working linings ofconverters in the oxygen conversion process for the production of steel.Dolomithic refractories are also widely utilized in the lining of rotaryDolomitic for the rock products industry.

Despite the high melting point of the CaO MgO mixture present indolomitic refractories, their use has been limited by the tendency ofthe oxides, especially lime, to revert to the hydroxide or carbonateform upon exposure to moisture or carbon dioxide in the atmosphere.

Various attempts have been made to improve the hydration resistance ofdolomitic refractories. The use of certain additives in relatively largeamounts (for example, over 1 percent by weight) such as clay, silica,iron oxide and other fluxing agents have been proposed to improve theirhydration resistance. However, the presence of such fluxing agents inquantities over 1 percent by weight has a deleterious effect on therefractoriness and particularly the density of the resulting product.Consequently, flux-containing dolomitic refractories are ofteninadequate to meet the demands of present day steel making processesespecially the oxygen conversion process which uses relatively highertemperatures to obtain greater melting and refining rates.

Another technique heretofore used for the preparation of dolomiticrefractories has involved admixing dolomite with pieces of coke andsintering the mixtures in shaft kilns. The combustion of the cokefurnishes the heat necessary for sintering. Unfortunately, the residoffluxing agents, the products produced by this process possess a loweredrefractoriness.

Accordingly, the search has been continued to find processes whichproduce dolomitic refractories with improved hydration propertieswithout adversely affecting refractory properties. Thus, it has beenproposed to calcine dolomite to produce dolomitic quicklime, an intimateadmixture of calcium oxide and magnesium oxide. Thereafter, thequicklime is hydrated under pressure until the calcium and magnesiumcontents are fully hydrated. The fully hydrated quicklime is formed intoa suitable refractory shape and sintered or dead-burned until thedesired refractory article is obtained.

This process makes it possible to prepare dolomitic refractories withoutthe addition of fluxing agents. However, the dolomite must be fullyhydrated and such hydration requires the use of pressure. As aconsequence, this process requires the use of special equipment andprocedures, such as autoclaving, which are expensive and time consuming,and render the economics of the process unfavorable. Further, thedeadburned dolomitic refractories obtained after fully hydrating thequicklime generally lack sufficient density and hydration stability tobe used effectively.

It has also been proposed to add lime or a low purity dolomite tomagnesium hydroxide and then burn this mixture to produce a syntheticclinker. Generally, it has been difficult to produce refractorymaterials having a desirable low porosity from synthetic clinkerproduced by this method. Further, unless high burning temperatures areprovided for bricks formed of such clinker, the hot modulus of rupturewould tend to be undesirably low.

One approach to stabilization of dolomitic refractories which has provedbeneficial is to produce a dense, high-purity dolomite grain having alow surface area. This low surface area makes the kinetics of thereaction converting CaO to the hydroxide or carbonate form much lessfavorable. Such a process is described in application Ser. No. 28,040,filed Apr. 13, 1970 by the assignee of this application, now U.S. Pat.No. 3,698,922. The method there disclosed is very desirable because nofluxing ingredients are added, and the refractory material retainsexcellent high temperature characteristics. This process yields aproduct which is still subject to hydration, but the high density of theproduct increases its useful shelf life by decreasing the rate at whichthe oxides are hydrated.

The present invention provides an improvement in a process of producinghydration resistant, low flux dolomitic refractory material in which adolomite containing at least 98 percent by weight of CaO and MgO iscalcined to produce quicklime, the quicklime is slaked to at leastpartially hydrate the oxides, and the slaked oxides are formed into arefractory shape and then sintered. The improvement comprisesincorporating in the slaked oxides at least 0.01 percent by weight on anoxide basis of a finely divided stabilizing agent such as A1 0 V 0aluminum fluoride, MgF,, and CaF and mixtures thereof to increase thetotal content of said agents to from 0.01 to about 1.0 percent by weighton an oxide basis.

It is surprising that the addition of such small amounts of stabilizingagents will produce significant changes in the hydration resistance ofhigh purity CaO MgO refractory grains. Besidesincreasing the hydrationresistance of low flux CaO MgO grain, small additions of the stabilizingagent also tend to greatly improve the appearance of the product.Samples of dolomite grain (CaO MgO) ordinarily appear nonhomogeneous anddisplay a mottled beige color. A small addition of M 0 for example,which still yields a material greater than 99 percent CaO Mgo produces avery homogeneous, almost white sample with no de tectable second phaseformation. The addition of small amounts, less than 1 percent. by-

general description and the following detailed description are exemplaryand explanatory, and should not be considered to restrict the scope ofthe invention.

Dolomitic refractories can be produced according to the presentinvention from most light burned, domestic, commercially available,low-flux dolomite rock compositions, that is, compositions containing atleast about 98 percent by weight of Ca and Mg on an oxide basis. It ispreferable, however, to employ a high purity dolomite rock, i.e., onecomprising at least 99 percent CaO and MgO. Such high purity dolomiterock provides products having greater refractoriness than rockcontaining a higher percentage of impurities.

The dolomite ore utilized in the process of the invention is in a lightburned condition. Light burning is accomplished by calcination of thedolomite at temperatures in the range of about 1,000 to 1,400 C. Thetime required for light burning will vary with the temperature used, andwill ordinarily be in the range of about one-half to 2 hours. Lightburning converts the carbonate in the dolomite to the oxide form, butdoes not cause the dolomite to agglomerate into large clumps.

Any suitable kiln, typically, a shaft or rotary kiln, can be utilizedfor light burning the dolomite. The fuel used in the kiln should be onethat does not introduce excessive amounts of impurities into thecalcined product. With rotary kilns, coals of moderate of low ashcontent, and gas or fuel oil are most effectively utilized. One type ofshaft kiln, shown as an Arnold kiln, is heated by means of coal-firedDutch ovens opening into the lower part of the kiln. On the other hand,Azbe type shaft kilns are preferably fired with produced or natural gas.

The size of the dolomite stone to be calcined is selected on the basisof the particular kiln being utilized, with the most efficient andeconomical use of the kiln being the prime criteria, rather than anyspecial process techniques.

As is well known, calcination of the raw dolomite stone produces amixture of oxides, predominantly CaO and MgO and popularly termedquicklime. The quicklime is slaked with water until it is at leastpartially hydrated. As used in the specification and claims, the termpartially hydrated means that the quicklime has been converted to amixture comprising calcium hydroxide, magnesium hydroxide and magnesiumoxide. Specifically, the term partially hydrated quicklime" as usedherein refers to compositions comprising from about 3 to 40 percent, andpreferably from 5 to about 20 percent by weight magnesium oxide (on anoxide basis). The balance of the partially hydrated quicklime is acombination of calcium hydroxide and magnesium hydroxide.

The quicklime is preferably partially hydrated by slaking the calcineddolomite in water at atmospheric pressure. At least 80 parts by weightof water are added for each 100 parts by weight of dolomite in thisslaking step. Preferably, from 100 to 150 parts of water are added per100 parts of dolomite.

The calcined dolomite may also be fully hydrated by autoclaving underpressure.

The preferred partial hydration of the quicklime is ordinarily completedin about one-fourth to 72 hours, depending on the particle size of themixed oxides, and the calcination temperature achieved, and can beaccomplished, for example, by placing the calcined dolomite and water ina pug mill. Preferably, the desired degree of hydration is achieved inabout one-half to hours. Achievement of the desired level of hydrationcan be determined by routine chemical analysis.

In accordance with the invention, at least 0.01 percent by weight on anoxide basis ofa finely divided stabilizing agent selected from the groupconsisting of Al- O V 0 aluminum fluoride, MgF and CaF and mixturesthereof is added to the slaked oxide to increase the total content ofstabilizing agent in the slaked oxide to from 0.1 to about 1.0 percentby weight on an oxide basis.

It has been found that the exact desired weight percent of stabilizingagent present in the slaked oxide varies with the stabilizing agentselected. From 0.25 to 1 percent of finely divided M 0 is desirablypresent in the material to be sintered. From 0.01 1.0 percent of V 0aluminum fluoride, MgF or CaF can be present in the material to besintered. These stabilizing agents are preferably present in amounts offrom 0.1 0.5 percent by weight.

The stabilizing agent has a particle size distribution such that atleast percent by weight passes a 200 mesh screen and in preferredembodiments all the sta bilizing agent passes a 325 mesh screen. Thissmall particle size distribution is important to insure that thestabilizing agent is uniformly distributed throughout the sintered grainproduced in accordance with the invention. The sintering step, ofcourse, tends to drive off fluorine, and fluorine has not been detectedin the sintered product.

It has also not been possible to detect in the sintered refractory grainthe presence, in a separate phase, of the A1 0 or V 0 stabilizing agentsafter they are incorporated in accordance with the present invention.

Preferably, the stabilizing agent is incorporated in the slaked oxidesas an aqueous slurry. This procedure for incorporating the stabilizingagent is preferred because more intimate mixing is achieved in slurryform. cases,

After the addition of the stabilizing agent, as set forth above, theresulting admixture is formed into a refractory shape suitable forsintering (dead burning) and is usually subsequently dried. In somecases a separate drying step is omitted and the small amount of dryingnecessary to provide abrasion resistance during firing is accomplishedduring the heating stage of the firing step.

The admixture may be nodulized, extruded, or pressed into pellets orbriquettes. The forming step generally involves a compaction or pressingoperation utilizing conventional equipment capable of supplyingsufficient pressure to give a coherent, dense refractory shape. Eitherhydraulically or mechanically actuated presses can be used. Pressures inthe range of about 4,500 to 10,000 psi have been found to besatisfactory, although higher pressures ranging up to approximately30,000 psi can be utilized. In some instances, the higher pressures arepreferable in order to obtain stronger refractory shapes possessingbetter resistance to abrasion during handling. Generally, it isdesirable that the water content of the admixture be such that thematerial shows a weight loss on drying of about 0-10 percent andpreferably 4-8 percent by weight.

It is possible to combine the drying and shaping steps of the presentprocess by extruding or nodulizing or a chain dryer.

The formed refractory shapes can be stored or immediately burned orsintered. It has been found that the refractory shapes tend to hardenbecause of the formaless otherwise noted, and all screen sizes are U.S.Sieve Series unless otherwise noted.

EXAMPLES 1-15 lomitic material of this invention, containing an added 5Sampl f Nia d l it f above 99,2 percent stabilizing agent, is sinteredor dead-burned. Sintering by weight CaO MgO are calcined at 1,000,l,200, or dead-burning 0f the formed r fr ry i omand 1,400C for 2 hoursand then slaked in water. The Pushed in y Suitable rotary or Shaft kilnwhich is slaked material is dried and pressed into disks at 10,000 Pableof pp y g the desired g temperatures The psi with 10 percent water as abinder and fired at from fuel utilized for firing the kiln may benatural gas, fuel 10 l,400 to 1,700 C for 2 hours. The resultingdensities oil, powdered coal or any other acceptable fuel. The andporosities a given i T bl I, use of natural gas or fuel oil is preferredsince these Table 1 also lists the condition of the samples which fuels,unlike coal, do not introduce additional impurihave been kept in aclosed vessel over water for 24 and ties into the product. 29 days in aroom kept at 72 F. Samples final fired at The dead-burning or sinteringtime varies, of course, 1,600" C for 2 hours are still intact after 24days in the with the temperature utilized. As might be expected,humidity chamber. The surfaces of all samplesexhibless sintering time isnecessary at higher temperatures. ited slight dusting. For example, ithas been found that exposure of a re- An x-ray analysis showed that thedolomite initially fractory shape for approximately .to 25 minutes atcalcined at 1,000 C and slaked in water consisted of about 1,700 Cproduces a sintered dolomitic refrac- 20 essentially all C3(OH)2 and g()z- That it was tory possessing relatively high density. Generally, thefully hydrated. The material calcined at 1,200 C and sinteringtemperature will be in the range of about then slaked contained MgO,Mg(OH) and Ca(OH) 1,300 to 1,800 C and preferably l,500 to 1,700 C. Thematerial calcined at 1,400 C and then slaked had After sinterng, therefractory product can be directly a larger amount of MgO present thanthe material calused or it can be pressed with pitch into brick shapes25 cined at 1,200 C, along with Ca(()H) and Mg(OH) and tempered byconventional means. Most often, the A chemical analysis of the materialsafter slaking is dolomitic refractory grains are coated with pitchbefore listed in Table 11. they are pressed into a final shape. Ofcourse, the sin- As seen in Table l, the hydration resistance followstered refractory product can also be pressed into a suitthe A120content. That is, the samples with the higher able brick shape and thenfired conventionally. A1 0 content seem to have the better hydrationresis- The incorporation of stabilizing agents in accordance tance. withthe invention can be used to improve the hydra- In the procedure ofExamples 13-15, varying tion resistance of natural dolomitic ores, andcan also amounts of A1 0 in excess of 0.25 percent by weight be used toimprove the hydration resistance of admixare added as an aqueous slurryto the dolomite compotures of a dolomite ore with a magnesium com oundsitions of Example 7 during the slaking step. Otherwise that willdecompose to form MgO, or light burned the procedures of Example 7 arefollowed. MgO. The incorporation of additional magnesium in As seeninTable 111, additions of A1 0 to the compodolomite ores is disclosed inUnited States patent applisitions improved the hydration resistance ofthe prodcation 129,205, filed Mar. 29, 1971, and assigned to ucts ofExamples 13-15 although the densitydecreased the assignee of thisapplication. The disclosure of applislightly. Several samples withincreased A1 0 contents cation Ser. No. 129,205 is incorporated hereinby refwithstood the hydration chamber for over three months erence.Thus, the dolomite grain produced in accorwithout cracking, and withonly very minor surface hy-t dance with the invention, can contain fromabout dration. 40-98 percent MgO, and from about 1-58 percent Each ofthe compositions containing added A1 0 CaO. can be fired at 1,600 C toproduce a dense sample of The density of the material produced inaccordance at least 3.35 g/cc without any cracking. with the presentinvention is very high, above about 3.3 The small Al O additions ofExamples 13-15 also g/c.c. after sintering. The porosity of the materialis greatly improved the appearance of the samplesQSamvery low. ples ofdolomite grain (CaO MgO) ordinarily appear For a better understanding ofthe invention, the folso non-homogeneous with a mottled beige color. Thelowing examples are provided. These examples areinsmall additions of A10,, of Examples 13-15, which still tended to be illustrative, and shouldnot be construed yield a material of at least about 99percent CaO MgO aslimiting the invention. All parts and percentages purity, result in veryhomogeneous, almost white samlistedin the specification and claims areby weight unples with no detectable A1 0 second phase formation.

TABLE I Calcining Firing Bulk Days in temp, temp., density, Porosity,humidity Condition of 0. C. g./cc. percent chamber sample 1,000 1,4003.23 2.5 24 Slight cracking. 1,000 1,500 3.38 0.131 29 Severely cracked.1, 000 1, 000 a. as 0. 05 24 1 Intact. 1,000 1,700 3.35 0.02 24 Slightcracking. 1,200 1,400 3.32 1.3 24 Powder. 1,200 1,500 3.38 0.04 29Slight cracking. 1,200 1,600 3. 39 0.05 24 Intact. 1,200 1,700 3.39 0.0524 Severely cracked. 1,400 1,400 3.37 0. 1 24 0. 1,400 1,500 3.39 0.0820 Slight cracking. 1,400 1,600 3.30 0.05 24 Intact. 1, 400 1, 700 s. 390.04 24 'Do.

TABLE II 16-22 containing an additive showed significantly im- Slakcridolomite Slakcddolomlte Slaked dolrpmite proved hydration resistancecompared to the control -100 mesh irac- 100 mesh lrac- 10O mcs iraction1000 c. ti0n1200" 0. tion 1400 c. conta'n'ng addmvecalclnc calcinecalcine (Exumples1-4) (Examples 6-8) (Examples 0-12 What is claimed 15:

percent percent percent by by weight by weight, weight 1. An improvementin a process of producmg hydra- 108 079 079 tion resistant, low fluxdolomitic refractory material in .522 .532 .235 which a dolomitecontaining at least 98 percent by I 030 551900 591060 910 weight of CaOand MgO is calcined to produce quick- M80 n 550 40480 40-520 1 0 lime,the quicklime is slaked to partially hydrate the ox- TABLE III WeightCalcining Firing Bulk percent Days in temp temp density, oi A1103humidity Condition 01 Ex. No. C gJcc. addition chamber sample 13 1, 2001, 600 3. 38 0. 3 90+ No cracking. 14 1,200 1,600 3.36 0.5 90+ Do. 151,200 1,600 3.35 1.0 00+ Do.

I EXAMPLES 16-22 2O ides, and the partially hydrated oxides are formedinto In these examples, the hydration resistance of a CaO MgO refractorygrain is improved by addition of magnesium fluoride and vanadiumpentoxide.

Samples are prepared containing 0, 0.25, 0.50, and 1.0 weight percentMgl additions to a hydroxide base composition containing 60 percent MgOand 40 percent CaO on an oxide basis. The sample containing no additiveserves as a control. A second series of samples are prepared containing0.01, 0.05, 0.10, and 0.50 by weight percent V 0 additions to the samebase composition. All of the batches are pressed at 15,000 psi and firedto a density greater than 3.3 g/cc with zero water absorption at 1,650 Cfor two hours.

The fired samples are then placed in a closed vessel over water at 72 Ffor hydration studies. The control sample with no addition ofstabilizing agent exhibited slight cracking after 20 days in the closedvessel. The sample containing 1.0 percent MgF, cracked after 20 days inthe humidity chamber. After 30 days in the chamber, the control sampleis still slightly cracked, as are the samples containing 0.01, 0.05, and0.10 percent V 0 and 0.125 and 0.25 percent MgF No evidence of dustingis apparent in any of the samples containing the added stabilizingagent.

After 75 days in the chamber: (a) the control sample is destroyed, (b)the sample containing 0.125 percent Mgl is only slightly cracked andstill in one piece and (c) the samples containing 0.01, 0.05, and 0.10percent V 0 are split in several sections, but show no evidence ofdusting. The samples with V 0, additions of 0.01 and 0.05 percentproduced better hydration resistance compared to samples with higher V0, additions. Also, the sample with 0.125 percent MgF exhibited betterhydration resistance than samples containing larger amounts of MgF,.Each of the samples (Examples a refractory shape and then sintered, theimprovement comprising; incorporating with the partially hydrated oxidesat least 0.01 percent by weight on an oxide basis of a finely dividedstabilizing agent selected from the group consisting of A1 0 V 0aluminum fluoride, MgF and CaF and mixtures thereof to increase thetotal content of said agents to from 0.01 to about 1.0.

percent by weight.

2. The process of claim 1 in which from 0.25 to 1.0 percent of finelydivided A1 0 is incorporated with the slaked oxides.

3. The process of claim 1 in which from 0.01 to 0.5 percent of finelydivided V 0 is incorporated with the slaked oxides.

4. The process of claim 1 in which from 01-1 .5 percent of aluminumfluoride is incorporated with the slaked oxides.

5. The process of claim 1 in which from 0.1-0.5 percent of MgF isincorporated with the slaked oxides.

6. The process of claim 1 in which from 01-05 percent of CaF isincorporated with the slaked oxides.

7. The process of claim 1 in which the finely divided stabilizing agenthas a particle size distribution such that at least percent by weightpasses a 200 mesh screen.

8. The process of claim 1 in which the dolomite contains at least 99percent by weight of CaO and MgO.

9. The process of claim 1 in which the slaked oxides are dried to amoisture content of about 0 to 10 percent by weight prior to beingformed into a refractory shape.

10. The process of claim 1 in which the stabilizing agent isincorporated in the slaked oxides as an aqueous slurry.

2. The process of claim 1 in which from 0.25 to 1.0 percent of finelydivided Al2O3 is incorporated with the slaked oxides.
 3. The process ofclaim 1 in which from 0.01 to 0.5 percent of finely divided V2O5 isincorporated with the slaked oxides.
 4. The process of claim 1 in whichfrom 0.1-1.5 percent of aluminum fluoride is incorporated with theslaked oxides.
 5. The process of claim 1 in which from 0.1-0.5 percentof MgF2 is incorporated with the slaked oxides.
 6. The process of claim1 in which from 0.1-0.5 percent of CaF2 is incorporated with the slakedoxides.
 7. The process of claim 1 in which the finely dividedstabilizing agent has a particle size distribution such that at least100 percent by weight passes a 200 mesh screen.
 8. The process of claim1 in which the dolomite contains at least 99 percent by weight of CaOand MgO.
 9. The process of claim 1 in which the slaked oxides are driedto a moisture content of about 0 to 10 percent by weight prior to beingformed into a refractory shape.
 10. The process of claim 1 in which thestabilizing agent is incorporated in the slaked oxides as an aqueousslurry.